Because the specific structures involved in the neurosurgical treatment of movement disorders are deep within the brain, stereotactic techniques are required to locate these targets.
What are stereotactic neurosurgical procedures?
The term “stereotactic” refers to the use of a three-dimensional coordinate system combined with an imaging technique, such as computed tomography (CT) scanning or magnetic resonance imaging (MRI), to precisely locate targets deep within the brain.
These techniques also are commonly used in other neurosurgical procedures, such as the biopsy of deep brain tumors. All of the procedures described below are performed using stereotactic techniques. The patient is awake during most or all of the procedure in order to monitor the response of the symptoms to the intervention, as well as to ensure that no undesired side effects are being produced.
In spite of being awake, patients generally tolerate the procedure well. A stereotactic head frame is employed to provide reference points for targeting. At the beginning of the procedure, the frame is attached to the patient’s head using local anesthetic to numb the scalp. An indicator box is then attached to the head frame, and an MRI or CT scan is obtained. Because of the indicator box, reference points, called fiducials, will surround the images of the brain and are used for precise targeting. After the calculations of the target coordinates have been completed, the patient is returned to the operating room for the remainder of the procedure.
In the operating room the patient is made as comfortable as possible on the operating bed. An incision is planned and anesthetized with local anesthetic. After the incision is made, the targeting arc is attached to the head frame. The combination of the targeting arc and head frame allows the precise localization of targets deep within the brain. A small hole is drilled through the skull, and a probe is directed towards the target.
For pallidotomy and subthalamic deep brain stimulation, microelectrode recording is performed to help verify the target. Microelectrode recordinginvolves the insertion of a very thin electrode to monitor the electrical activity of neurons (nerve cells). The pattern of electrical activity is different within different structures of the brain, and therefore can provide confirmation that the electrode is within the desired structure.
For tremor surgeries like thalamotomy and thalamic deep brain stimulation, the effect of electrical stimulation on the patient’s tremor is usually dramatic and provides adequate confirmation of the target. Once the target has been confirmed, a small area of tissue may be destroyed, as in the case of thalamotomy and pallidotomy, or an electrode may be inserted, as with thalamic or subthalamic deep brain stimulation.